The performance required of tires for vehicles, conveyor belts and other industrial rubber articles has become increasingly versatile. In the tires for vehicles, radial tires are extensively used not only as tires for passenger cars but also as tires for large cars such as trucks and buses, and the useful life of tires has been remarkably extended by several reasons including the advances in the mixing technology of tread rubber. Large tires that have been used for large cars such as trucks and buses are retreaded after the end of their primary life so that they are put to service for the secondary, and even tertiary, use. These uses sometimes extend to several hundreds thousand kilometers and the belts and carcasses of tires on vehicles are required to perform satisfactorily for this period.
In the above radial tires, rubber-coated steel cords are used to form the belts or carcasses in order to improve the steering stability, structural durability, wear resistance and puncture resistance of the tires. With such tires, the adhesion between rubber material and metal material can be a problem.
Rubber material normally adheres to metal material, when sulfur mixed in the rubber material during vulcanization, reacts with copper in the plating on the metal material so as to form a sulfide at the rubber-metal interface. When a tire is rolling on the road, hysteresis loss of the rubber material generates heat, and if the generated heat is large enough, adhesion between the rubber material and the metal material is broken, and the coated rubber separates from the steel cord and this "separation" renders further rolling of the tire on the road impossible.
Further, the tire tread or sidewall may sometimes be damaged when the tire is rolling on the road. If the damage is deep enough to reach the steel cord, moisture that has permeated the tire through the damaged part will be vaporized by the heat generated during tire rolling on the road, and the vapor (getting into the space between filaments in the steel cord) will destroy the adhesion between the steel cord and the coated rubber, thereby inducing "separation".
A further problem will occur if the tire is inflated with moisture containing air. The moisture will penetrate through innerliner of the tire to reach the steel cord and this may induce "separation" by breaking the adhesion between the coated rubber and the steel cord.
With a view to avoiding these phenomena, various techniques have been proposed that are chiefly intended to improve the adhesion between the rubber and the steel cords. One of the techniques proposed concerns the compounding techniques of rubber material with which steel cords are coated. It was discovered as early as in the nineteen-fifties that the adhesion between the rubber and the steel cords could be improved by incorporating cobalt salts of organic acids in the rubber material, and many studies have been conducted regarding the quantity and type of such the cobalt salts to be added. For example, JP-A-60-42440 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application") shows that improved adhesion can be attained by optimizing the quantities of sulfur and cobalt salts of organic acids. Another approach has been to use the HRH based composition mixing white carbon, resorcin and hexamethylenetetramine. Drawbacks of the above cobalt method include that if cobalt salts of organic acids are added in large amounts, unvulcanized rubber will be deteriorated, or the deterioration of adhesive power due to thermal aging will occur, either on account of hot or prolonged vulcanization or rolling of the tire on the road for a distance of many hundred kilometers. The HRH composition's principal drawbacks are that they cause problems during production such as environmental pollution and rubber scorching.
The second approach that has been taken to improve the adhesion between the rubber and the steel cords is directed at the plating layer to be deposited on a metal material. JP-B-51-8389 (the term "JP-B" as used herein refers to an "examined Japanese patent publication") and JP-A-55-105548 disclose a plating layer containing nickel. Alloy platings containing three or four metals are disclosed in JP-A-55-45884, JP-A-55-71887,JP-A-55-105548, JP-A-56-82604, JP-A-54-89940, JP-A-61-243194, JP-A-61-72545, etc.